
Answer:
I recommend reading this article from the Scott Pilgrim Wiki. It does a better job of explaining than I can, but if you don’t want to read all that I’ll try my best to give a condensed version. Ahem.
Subspace is like an alternate reality that can only be accessed in certain ways (the doors with stars on them, Ramona’s bag, etc). When you open up a door into Subspace, distances are usually “compressed” so a place that might be miles away in the real world is only a short walk inside Subspace. This is why Ramona uses it to deliver packages; makes sense right? The reason why Ramona’s bag can hold so many things is because the space inside her bag is expanded, rather than compressed, sort of like the opposite of the doors. There could be a whole room inside there (and there probably is) filled with all of her stuff!
I don’t know if that answers all your questions, but like I said, check out the SP Wiki page on it. It gets into a lot more detail than I do. :]
#subspace